Photonic crystal optical element and manufacturing method therefor

ABSTRACT

To provide a novel technique for producing a difference in refractive index between two regions. An optical element according to the present invention includes a first porous region ( 2002 ), a second porous region ( 2004 ), and a non-porous region ( 2003 ) formed between the first porous region ( 2002 ) and the second porous region ( 2004 ), the non-porous region having a refractive index higher than a refractive index of the first porous region.

TECHNICAL FIELD

The present invention relates to an optical element that is applicablein association with electromagnetic waves including a visible light, aterahertz wave, a microwave, and an X-ray, and a manufacturing methodtherefor. The present invention also relates to an optical element usedin association with an optical communication and an informationprocessor using light, and a manufacturing method therefor.

BACKGROUND ART

In recent years, an optical element categorized as a photonic crystal(PC) has attracted an attention.

This is mainly owed to techniques in which a periodic structure of anoptical material is formed to produce a periodic refractive indexprofile caused by a difference in refractive index, thereby makingeffective use of behavior of light in such a specific refractive indexprofile, or making effective use of such a phenomenon that, when thestructure having a specific refractive index profile includes a lightemitting material etc., its light emission state is controlled (see, E.Yablonovitch “Phys. Rev. Lett.” Vol. 58, p. 2059, 1987). Heretofore, anapplicability of an optical element based on those techniques has becomea controversial subject.

Regarding the optical element techniques, hitherto, a so-calleddistributed feedback (DFB) laser attained by exploiting aone-dimensional periodic structure to a semiconductor laser has alreadycome into practical use. This may be called an optical element to whicha one-dimensional photonic crystal is applied.

Further, in recent years, there has been an impressive development on alaser element, a so-called vertical cavity surface emitting laser(VCSEL) as well. As shown in FIG. 18, however, in this element,light-confinement in a light emitting direction, i.e. a directionvertical to a substrate surface, is effected by two multilayered filmmirrors 1801 and 1802 made up of a porous compound semiconductor formedthrough epitaxial growth, while light-confinement in a horizontaldirection with respect to the substrate surface is realized according tothe principle that total reflection at the boundary between acylindrical semiconductor and the air allows light-confinement; this isbecause the entire cylindrical semiconductor has a higher refractiveindex than its surrounding substance (air) like a core of an opticalfiber.

In general, well known as a manufacturing method for the multilayeredfilm mirror is a deposition or sputtering method. In the case of aimingat ones having a high quality, small loss, and high matching propertywith an active layer (light emitting material layer) like a surfaceemitting laser, an epitaxial growth technique capable of producing acrystal having a monolithic structure or a substantially monolithicstructure over multiple layers is inevitably used.

Meanwhile, there has been known regarding the multilayered film mirroranother monolithic manufacturing method aside from the epitaxial growthtechnique, for example, a technique disclosed in Lehman Reece et al.“Applied Physics Letters”, Vol. 81 (2002), pp. 4895. The disclosedtechnique is such that a silicon substrate is anodized in an HFsolution, and an intensity of an electric field applied at this time ismodulated periodically with respect to time, whereby two layers havingdifferent porosities are alternately formed. The silicon that has beenmade porous through anodization maintains its crystallinity, i.e.monolithic quality, even after turned porous since the substrate isoriginally made of single crystal. Also, as disclosed in the document,low-temperature anodization affords a more uniform interfacial structurebetween layers, so a multilayered film of optical quality can be formed.

Nowadays, the most popular one among study cases that are being reportedas the “photonic crystal” is a two-dimensional photonic crystal formedby a process of patterning a two-dimensional periodic structure to aslab-shaped semiconductor etc. In particular, there are made extensivestudies on the basic principle of application of the two-dimensionalphotonic crystal having a two-dimensional structure of cylindrical poresto optical communication parts, for example.

In such a two-dimensional photonic crystal, a light-confinementperformance in one non-periodic direction (in general, in a thicknessdirection) may be inferior to those in the remaining two periodicdirections in terms of light confinement under control. In contrast,some attempts have been made to obtain a periodic structure in all ofthe three directions, i.e. a so-called three-dimensional (3D) photoniccrystal structure.

As for the three-dimensional photonic crystal developed so far, forexample, there is an element called a double-cross or wood-pile typeelement, which is manufactured by a laminating method (see, Noda“Photonic Crystal Technique and its Application” p. 128, 2002, CMCpublishing, Co. Ltd.). Given as another example thereof is an elementmanufactured by a micromechanics manufacturing method (see, Hirayama etal. “Photonic Crystal Technique and its Application” p. 157, 2002, CMCpublishing, Co., Ltd.). Besides, given as still another example thereofis an element manufactured by a thin-film lamination growth methodcalled self-cloning (Japanese Patent Publication No. 3325825 B or Sato“Photonic Crystal Technique and its Application” p. 229, 2002, CMCPublishing).

DISCLOSURE OF INVENTION

The present invention provides novel means for producing a difference inrefractive index between two regions.

In the present invention, as the difference in refractive index betweentwo regions, a difference in refractive index between a porous regionand a non-porous region is utilized.

More specifically, an optical element according to one aspect of thepresent invention includes: a first porous region; a second porousregion; and a non-porous region formed between the first porous regionand the second porous region, the non-porous region having a refractiveindex higher than a refractive index of the first porous region.

Also, in the optical element according to the present invention, a firstlayer including the first porous region may be formed, a second layerincluding the second porous region may be formed, and a third layerincluding the non-porous region may be formed between the first layerand the second layer and have a region with a refractive index differentfrom the refractive index of the non-porous region, in its in-planedirection.

Note that the region in the third layer with the refractive indexdifferent from the refractive index of the non-porous region may have aporous structure.

Also, the non-porous region may function as an optical waveguide, and aspacer layer of a porous structure may be formed between the secondlayer and the third layer.

Here, the formed spacer layer may include a plurality of regionsdifferent in refractive index, in its in-plane direction, for example.

Also, it is possible to provide an information processor, including: theoptical element; and a light emitting portion, for example.

Also, in order to solve the afore-mentioned problems, according toanother aspect of the present invention, there is provided a monolithicoptical element, including: at least one porous layer having a pore sizesmaller than a light wavelength; and at least one crystal layer having arefractive index higher than a refractive index of the porous layer, theporous layers and the crystal layers being laminated on one another, inwhich the crystal layer having the higher refractive index is of asingle crystal structure over a plurality of layers thereof, and two ofthe porous layers sandwiching the crystal layer having the higherrefractive index are not connected through a hole.

Also, according to another aspect of the present invention, there isprovided a method for manufacturing a monolithic optical element,including the steps of: turning porous a surface of a crystal layer; andconducting epitaxial growth of a crystal layer from the porous surface,the turning and promoting steps being repeated to laminate the porouslayer and the crystal layer.

Also, the method for manufacturing a monolithic optical elementaccording to the present invention is applicable to a method formanufacturing an optical element described below, for example.

A laser can be obtained by introducing a laser medium into apredetermined layer in the laminated structure (see Example 5 describedlater). The predetermined layer in the laminated structure may have anon-periodic pattern in a predetermined in-plane position, and the lasermedium may be introduced in the periodic pattern position and itsvicinity. It is possible to introduce a non-linear optical medium tocompose a high-efficiency optical switch in the same way that the lasermedium is introduced to compose the laser.

Further, the present invention can provide an optical waveguide byinterposing a non-porous region between porous regions.

Also, according to the present invention, it is possible to realize aninterchange of thin waveguides or microring resonators using a core andclad with a large difference in refractive index by repeatinganodization for forming a porous portion, porous patterning (porouspattern formation), and epitaxial growth.

Also, a monolithic waveguide coupling optical element according to thepresent invention, which includes at least one porous layer having apore size smaller than a light wavelength; at least one crystal layerhaving a refractive index higher than a refractive index of the porouslayer, the porous layer and the crystal layer being laminated to threeor more layers on a substrate comprises at least one optical waveguidefor propagating a light; and a waveguide coupling portion forselectively coupling light propagation paths for propagation of light indifferent directions in the optical waveguides, in which the opticalwaveguide has a core made up of the crystal layer having the higherrefractive index, and a clad made of a porous material, and the crystallayer having the higher refractive index is of a single crystalstructure over a plurality of layers thereof.

Here, the waveguide coupling portion is a resonator, for example.

Also, in the waveguide coupling optical element, the waveguide and theresonator sandwich a spacer layer.

The spacer layer may be formed of a porous material, and the porosity ofthe porous material may be controlled.

Also, a method for manufacturing an optical element according to thepresent invention includes the steps of: forming a porous layer having apore size smaller than a light wavelength on a substrate; forming acrystal layer having a refractive index higher than a refractive indexof the porous layer on the porous layer through epitaxial growth; andturning porous a part or all of a surface of the crystal layer formedthrough the epitaxial growth.

Now, the property meant by the term “single crystal structure” referredto in the present invention will be described below.

In the present invention, the “single crystal structure” is assumed tobe “actual single crystal”. To be specific, even if any defectsinevitably involved under actual conditions, that is, non-uniformcrystallinity and variation in crystal orientation are locally caused,any crystal can be called crystal having a single crystal structure inprinciple insofar as the crystal is produced by a crystal growth methodor a manufacturing method where manufacturing conditions are controlledwith a view to producing single crystal.

Also, even a porous crystal may be assumed to have the single crystalstructure as far as a portion other than pores of the porous crystalmaintains its crystallinity. For example, a single crystal made porousthrough anodization can be assumed to have the single crystal structure.Further, in a case of conducting epitaxial growth of a non-porouscrystal continuously from the porous surface, a porous layer andnon-porous layer can be assumed to form a single crystal together(continuously).

Also, the term “monolithic” referred to in the present invention meansnot only an optical element structure having plural refractive indexprofiles, which takes on the single crystal structure in its entirety,but also a structure that takes on the single crystal structure but ispartially made amorphous through oxidation.

In such a case, to discuss a concern about whether or not the structurehas the single crystal structure, the entire crystal portion excludingthe amorphous portion does not lose its original crystallinity orcrystal orientation even if a little translational or angular deviationoccurs in its entirety. In this case, it can be assumed to maintain thesingle crystal structure. Note that the amorphous portion is out of acategory of crystal and thus, needless to say, does not have the singlecrystal structure.

To give specific examples of the above, for example, in a case ofthermally oxidizing a porous silicon portion of a monolithicmultilayered film having the single crystal structure in its entiretyand formed by laminating a silicon crystal layer and a porous siliconcrystal layer, into an amorphous form (porous SiO₂), even when thesilicon crystal undergoes separation between different layers in themultilayered film through the amorphous layer, the original structurethereof is the single crystal structure, so the silicon crystal layercan be assumed to have the single crystal structure throughout themultilayered film even with a little translational or angular deviationin its entirety.

Meanwhile, to give an example of a non-monolithic one, for example,after separately forming a thin film of the single crystal structure anda multilayered film, these are bonded through wafer bonding etc. forlamination. In such a case, the resultant laminate is not regarded asmonolithic. Besides, in this case, a crystal region extending across thebonded surface does not, needless to say, have the single crystalstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D and 1E are schematic diagrams each showing amanufacturing method for a one-dimensional photonic crystal using Siaccording to Example 1 of the present invention;

FIGS. 2A, 2B, 2C, 2D, 2E, 2F and 2G are schematic diagrams each showinga manufacturing method for a one-dimensional photonic crystal includingporous SiO₂ according to Example 2 of the present invention;

FIGS. 3A, 3B, 3C and 3D are schematic diagrams each showing a structureexample of a three-dimensional photonic crystal according to Example 3of the present invention;

FIGS. 4A, 4B, 4C, 4D, and 4E are schematic diagrams each showing astructure example of a three-dimensional photonic crystal according toExample 4 of the present invention;

FIGS. 5A, 5B, 5C, 5D, 5E, 5F and 5G are schematic diagrams each showingan in-plane patterning method for anodization according to Example 4 ofthe present invention;

FIGS. 6A, 6B, 6C and 6D are schematic diagrams each showing a structuralexample (top view) of a pattern in each layer according to Example 4 ofthe present invention;

FIG. 7 is a schematic diagram showing a structural example (sectionalview, bird's-eye view) of a pattern in each layer according to Example 4of the present invention;

FIGS. 8A, 8B, 8C and 8D are schematic diagrams each showing anotherstructural example (top view) of the pattern in each layer according toExample 4 of the present invention;

FIG. 9 is a schematic diagram showing another structural example(sectional view, bird's-eye view) of the pattern in each layer accordingto Example 4 of the present invention;

FIGS. 10A, 10B and 10C are schematic diagrams each showing anotherstructural example (top view) of the pattern in each layer according toExample 4 of the present invention;

FIG. 11 is a schematic diagram showing another structural example(sectional view, bird's-eye view) of the pattern in each layer accordingto Example 4 of the present invention;

FIGS. 12A and 12B are schematic diagrams each showing an example ofapproximating a curved member with multiple thin-film structuresaccording to Example 4 of the present invention;

FIG. 13 is a schematic sectional diagram showing an example of ananodization method;

FIG. 14 is a schematic sectional diagram showing an example of a batchanodization method for plural wafers;

FIGS. 15A and 15B are schematic diagrams each showing a structureexample of a photonic crystal laser according to Example 5 of thepresent invention;

FIG. 16 is a schematic diagram showing another structure example of thephotonic crystal laser according to Example 5 of the present invention;

FIG. 17 is a schematic diagram showing still another structure exampleof the photonic crystal laser according to Example 5 of the presentinvention;

FIG. 18 is a schematic diagram showing a structure of a conventionalsurface emitting laser;

FIGS. 19A, 19B and 19C are schematic diagrams each showing an example ofa three-dimensional photonic crystal according to Example 4 of thepresent invention;

FIG. 20 is a schematic sectional diagram for explaining the presentinvention;

FIG. 21A is a bird's-eye view schematically showing an optical elementaccording to Example 6 of the present invention, FIG. 21B is a top viewthereof, and FIG. 21C is a sectional view thereof;

FIGS. 22A, 22B, 22C, 22D, 22E, 22F, 22G and 22H are schematic diagramseach showing a manufacturing method for the optical element according toExample 6 of the present invention;

FIG. 23 is a schematic diagram showing an example of a method of makinga crystal porous through anodization according to Example 6 of thepresent invention;

FIG. 24 is a schematic diagram showing another example of the method ofmaking the crystal porous through the anodization according to Example 6of the present invention;

FIGS. 25A, 25B, 25C, 25D, 25E, 25F and 25G are schematic diagrams eachshowing an example of an in-plane patterning method for anodizationaccording to Example 6 of the present invention;

FIG. 26 is a schematic diagram showing an example of an optical circuitwhere plural elements are provided in plane according to Example 7 ofthe present invention; and

FIGS. 27A and 27B are schematic diagrams each showing a modified exampleof Example 6 of the present invention, and FIGS. 27C and 27D areschematic diagrams each showing another modified example of Example 6 ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

First Embodiment Mode

Referring to FIG. 20, an optical element according to the presentinvention will be described.

In FIG. 20, reference numeral 2002 denotes a first porous region; 2003,a non-porous region; and 2004, a second porous region.

Here, the non-porous region 2003 has a refractive index higher than thatof the first porous region.

In this way, the present invention makes use of a structural differencelike a difference between the porous structure and the non-porousstructure to produce a difference in refractive index between tworegions.

Note that in FIG. 20, reference numeral 2102 denotes a first layerincluding the first porous region 2003; 2103, a third layer includingthe non-porous region 2003; and 2104, a second layer including thesecond porous region.

Regions 2202 and 2302 sandwiching the first porous region 2002 in thefirst layer 2102 may have either the same structure as that of the firstporous region 2002 or the non-porous structure. In other words, arefractive index profile pattern realized by the porous structure andthe non-porous structure may be formed in an in-plane direction of thefirst layer 2102, or a periodic structure may be formed in the in-planedirection.

Regions 2204 and 2304 sandwiching the second porous region 2004 in thesecond layer 2104 may have either the same structure as that of thesecond porous region 2004 or the non-porous structure. In other words, arefractive index profile pattern realized by the porous structure andthe non-porous structure may be formed in an in-plane direction of thesecond layer 2104, or a periodic structure may be formed in the in-planedirection.

Regions 2203 and 2303 sandwiching the non-porous region 2003 in thethird layer 2103 may have either the same structure as that of thenon-porous region 2003 or the porous structure. In other words, thethird layer 2103 may have a region having a refractive index differentfrom that of the non-porous region 2003, in its in-plane direction.Also, a refractive index profile pattern realized by the porousstructure and the non-porous structure may be formed in the in-planedirection of the third layer 2103, or a periodic structure may be formedin the in-plane direction.

In addition, another layer may be interposed between the first layer2102 and the third layer 2103 or between the third layer 2103 and thesecond layer 2104.

Further, the third layer 2103 may be comprised of plural layers.

Note that it is preferable to set the refractive index of the non-porousregion 2003 higher than those of the first porous region 2002 and secondporous region 2004.

Also, the thicknesses of the first layer 2102 and/or the second layer2104 may be set smaller or larger than that of the third layer 2103.

Furthermore, in FIG. 20, a description has been made using threeregions, i.e. the two porous regions 2002 and 2004 and the onenon-porous region 2003. It is possible to laminate additional pluralnon-porous regions and porous regions on the second porous region 2004.

Second Embodiment Mode

A description will be given later in Examples 7 and 8 of the presentinvention, of an optical element so configured as to optically couple athin waveguide with a microring resonator as an optical waveguideutilizing a difference in refractive index between the porous region andthe non-porous region.

Here, each of reference symbols in the accompanying drawings isexplained.

Reference numerals 101, 201, 401, 501, and 504 denote an Si substrate(single-crystal substrate).

Reference numerals 102, 202, 402, 503, 507, 601, and 603 denote a poroussilicon layer.

Reference numerals 103, 204, and 403 denote an epitaxial growth Silayer.

Reference numerals 203, 207, and 305 denote a porous SiO₂ layer.

Reference numeral 304 denotes periodically arranged through-holes.

Reference numeral 502 denotes a resist pattern.

Reference numeral 505 denotes a microelectrode.

Reference numeral 506 denotes a wiring.

Reference numeral 1201 denotes a curved structure to be approximated.

Reference numeral 1202 denotes a porous silicon having a curvedstructure approximated using plural layers.

Reference numeral 1203 denotes an epitaxial growth silicon.

Reference numeral 1301 denotes an Si substrate for manufacturing aphotonic crystal.

Reference numeral 1304 denotes a Pt-made surface electrode.

Reference numeral 1305 denotes a lower supporting member.

Reference numeral 1306 denotes an upper supporting member.

Reference numeral 1307 denotes an anode.

Reference numeral 1308 denotes a Pt mesh electrode.

Reference numeral 1309 denotes a cathode.

Reference numeral 1402 denotes a holder.

Reference numerals 1403 and 1303 denote an O-ring.

Reference numeral 1404 denotes a suction portion.

Reference numerals 1405 and 1302 denote an HF solution.

Reference symbols 1406 a and 1406 b denote a platinum electrode.

Reference numeral 1408 denotes an HF solution tank (anodization tank).

Reference numeral 1409 denotes a holder groove.

Reference numeral 1501 denotes a lower three-dimensional photoniccrystal.

Reference numeral 1502 denotes an upper three-dimensional photoniccrystal.

Reference numeral 1503 denotes a laser medium layer.

Reference numeral 1504 denotes an in-plane point defect.

Reference numeral 1601 denotes a three-dimensional photonic crystal.

Reference numeral 1602 denotes a laser medium.

Reference numerals 1701 and 1702 denote a one-dimensional photoniccrystal.

Reference numeral 1703 denotes a resonator spacer layer.

Reference numeral 1704 denotes a laser medium active substance.

Reference numerals 1801 and 1802 denote an epitaxial growth multilayeredfilm mirror.

Reference numeral 1803 denotes a resonator spacer layer.

Reference numeral 1804 denotes a laser medium active layer.

Reference numeral 1901 denotes a porous silicon.

Reference numeral 1902 denotes an epitaxial growth silicon.

Reference numeral 1903 denotes a porous SiO₂.

Reference numeral 1904 denotes the air.

Reference numeral 21101 denotes a silicon (single-crystal) substrate.

Reference numeral 21102 denotes a porous silicon clad layer.

Reference numeral 21103 denotes a porous silicon waveguide layer.

Reference numerals 21104, 701, and 704 denote a silicon waveguide(core).

Reference numerals 21105, 702, and 705 denote a silicon waveguide(core).

Reference numeral 21106 denotes a porous silicon spacer layer.

Reference numeral 21107 denotes a porous silicon resonator layer.

Reference numerals 21108, 703, and 706 denote a silicon microringresonator (core).

Reference numeral 21109 denotes an incident light.

Reference numerals 21110 and 21111 denote an outgoing light.

Reference numeral 22201 denotes an epitaxial growth silicon layer(waveguide layer).

Reference numeral 22202 denotes an epitaxial growth silicon layer(spacer layer).

Reference numeral 22203 denotes an epitaxial growth silicon layer(resonator layer).

Reference numeral 23301 denotes an Si substrate.

Reference numeral 23302 denotes an HF solution.

Reference numeral 23303 denotes an O ring.

Reference numeral 23304 denotes a Pt surface electrode.

Reference numeral 23305 denotes a lower supporting member.

Reference numeral 23306 denotes an upper supporting member.

Reference numeral 23307 denotes an anode.

Reference numeral 23308 denotes a Pt mesh electrode.

Reference numeral 23309 denotes a cathode.

Reference numeral 24401 denotes an Si substrate.

Reference numeral 24402 denotes a vacuum chuck pipe.

Reference numeral 24403 denotes a wafer holder.

Reference numeral 24404 denotes an HF solution tank.

Reference numeral 24405 denotes an HF solution.

Reference numeral 24406 denotes an anode.

Reference numeral 24407 denotes a cathode.

Reference numeral 25501 denotes a silicon substrate.

Reference numeral 25502 denotes a resist pattern.

Reference numeral 25505 denotes a microelectrode.

Reference numeral 25506 denotes a wiring.

Reference numeral 26601 denotes a waveguide.

Reference numeral 26602 denotes a microring resonator.

Reference numeral 26603 denotes a spacer layer in a first region(perspective diagram).

Reference numeral 26604 denotes a spacer layer in a second region(perspective diagram).

Reference numerals 26605 to 26611 denote spacer layers in third to fifthregions (perspective diagram).

EXAMPLES

Hereinafter, the present invention will be described in more detailbased on examples. However, the present invention is not limited tothese examples.

Example 1

This example relates to a photonic crystal (multilayered film structure)produced by a method of repeating epitaxial growth and anodizationaccording to the present invention, and a one-dimensional photoniccrystal (multilayered film structure) produced by carrying out a methodfor manufacturing a nanophotonic element. In particular, this examplerelates to a photonic crystal using silicon as a material therefor.

Referring to FIGS. 1A to 1E, a manufacturing method for a multilayeredfilm structure according to this example will be described.

On a surface of a silicon substrate 101 (single-crystal substrateobtained through crystal growth by a Czochralski method etc.) as shownin FIG. 1A (Step(1)), a porous silicon layer 102 is formed throughanodization as shown in FIG. 1B (step (2)). Note that in the followingdescription, the anodization in the present invention is desirablyeffected with an electrolyte at a low temperature of 0° C. or lower, forexample, about −20° C. This is because a boundary surface of the porouscrystal resulting from the anodization is made smooth to prevent anoptical loss, i.e. light scattering etc. Also, the term “porous” used inthe present invention means one with a pore size of about 1/100 orsmaller of a wavelength of light used; for example, with respect to thelight having a wavelength of 1.5 μm, a so-called Mesoporous-Si orMicroporous-Si having a pore size of 10 nm or smaller is formed.

FIG. 13 is a schematic diagram showing a structure example of a devicefor turning a wafer surface into a porous layer through anodization. InFIG. 13, a wafer 1301 is held such that its surface is immersed in a HFsolution 1302. The holding operation is carried out by a lowersupporting member 1305 and an upper supporting member 1306 through theintermediation of an O-ring 1303 and a Pt surface electrode 1304. Theupper supporting member 1306 is provided with a HF solution tankcommunicating with the wafer 1301, which is filled with the HF solution1302. A Pt mesh electrode 1308 is placed in the HF solution 1302. The Ptsurface electrode 1304 and the Pt mesh electrode 1308 are connected toan anode 1307 and a cathode 1309, respectively. A predetermined electricfield is applied to the surface of the wafer 1301 through the HFsolution 1302 on the side of the cathode 1309 and through the substrateon the side of the anode 1307 for carrier injection.

Note that the structure for anodization is not limited to that of thisexample but generally-employed various methods can be used asappropriate.

Anodization conditions are listed below:

-   Starting wafer: P+(100)Si 0.01 Ω^(−cm)-   Solution: HF, C₂H₅OH, and H₂O-   Anodization current: 150 mA/cm³

As regards an anodization method, it is also possible to collectivelyprocess the plural wafers with a device structure shown in FIG. 14.Hence, this step can be preformed at a low cost.

For example, it is also possible to collectively process plural wafers1401 with a device structure shown in FIG. 14. In FIG. 14, referencenumeral 1402 denotes a wafer holder; 1403, an O-ring; 1404, a suctionportion; 1405, a HF solution; 1406 a and 1406 b, platinum electrodes;1408, an anodization tank; and 1409, a holder groove. Further, thestructure for anodization is not limited to that of this example butgenerally-employed various methods can be used.

Also, the pore size, density, and thickness of the porous silicon layercan controlled over a wide range depending on the composition of theanodization solution, the anodization current, the conduction type ofthe substrate, and the conductivity thereof. Also, platinum having anextremely high resistance to hydrofluoric acid or a metal covered withplatinum on its surface is used as an electrode. In the case ofcollectively anodizing the plural wafers into porous layers, as shown inFIG. 14, the anodization solution itself in contact with both surfacesof the wafer acts as an electrode. This enables uniform contact andenhances controllability of a porous layer to be formed.

In a subsequent step, i.e. step (3) of FIG. 1C, epitaxial growth ofsilicon is allowed to proceed starting with a silicon surfaceconstituting the surface of the porous silicon layer 102 formed in step(2). In this way, an epitaxial growth silicon layer 103 is formed. Theepitaxial growth silicon layer 103 has a thickness about twice largerthan that of the porous layer but, as described later, is preciselycontrolled depending on the thickness ratio (duty) between the siliconlayer and the porous silicon layer in the multilayered film.

As for the epitaxial growth, it is well known that the porous layerformed with the above method maintains a crystal orientation of asingle-crystal substrate, and a uniform, pore-free single crystal layercan be formed thereon through epitaxial growth.

A description will be given of conditions for forming the epitaxialgrowth Si layer 103 by chemical vapor deposition (CVD) etc. through theepitaxial growth on the porous Si layer 102. First, it is important toeffect the epitaxial growth in a hydrogen atmosphere. It promotessealing of the pores on the porous layer surface and enables theformation of a high-quality epitaxial layer thereon (Yonehara et al.,“Appl. Phys. Lett.”, Collective Report, September, 2002).

Epitaxial growth conditions are listed below:

-   Vapor deposition Temperature: 1,000° C.-   Gas: SiH₄/H₂-   Pressure: 700 Torr

In a subsequent step, i.e. step (4) of FIG. 1D, the surface of theepitaxial growth silicon layer 103 formed in step (3) is made porousthrough the anodization. Note that the anodization is only allowed toproceed up to a given thickness from the surface; in short, part of theepitaxial growth silicon layer remains. The thicknesses of the epitaxialgrowth silicon layer and porous layer are designed according to a targetoptical element, and a light wavelength or light incident angledistribution, and may be changed for each layer. Here, the design isbasically made in consideration of the refractive index of the siliconlayer and the refractive index depending on the porosity of the poroussilicon layer. Assuming that the refractive index of silicon is 3.5, theeffective refractive index of porous silicon is 2.5, and the lightwavelength adopted is 1.5 μm, a typical layer thickness is such that theoptical path length in each layer, i.e. the length derived bymultiplying the refractive index by the layer thickness, equals ¼ of thelight wavelength, leading to the condition that the silicon layer is 0.1μm in thickness, and the porous silicon layer is 0.15 μm in thickness.

As a subsequent step, if the processes of steps (3) and (4) are furtherrepeatedly carried out on the structure completed up to step (4) in thisway, and one epitaxial growth silicon layer and one porous silicon layerare added in one cycle, a multilayered film structure 105 is obtained(FIG. 1E).

The multilayered film structure 105 is made up of a multilayered filmincluding silicon and porous silicon and has such an advantage that adifference in refractive index between layers can be increased ascompared with a conventional multilayered film where the porosity ofporous silicon is modulated. Hence, in manufacturing a high-reflectancemirror or Fabry-Perot resonator using the multilayered film, it ispossible to reduce the number of layers and realize reduction in size ofan element and in cost for a high-cost laminated structure obtainedthrough epitaxial growth. Also, in the multilayered film structure 105realized with the manufacturing method according to the presentinvention, the porous silicon can maintain crystallinity such as thecrystal orientation and lead to the epitaxial growth silicon. Thus, themonolithic structure can be attained in its entirety. Unlike apolycrystalline silicon or amorphous silicon, this solves the problemthat, when functioning as an optical element, light is scattered due toinfluence of a grain boundary etc. to hinder the realization of ahigh-quality optical element.

For example, if the above technique of the background art is used tomanufacture the multilayered film mirror through lamination based on theepitaxial growth, basically, there can be only such a small differencein composition ratio that can be achieved through the epitaxial growth.Thus, regarding an optical element requiring a relatively highrefractive index difference between layers, it is difficult to increasethe refractive index difference. This means that the number of layersshould be inevitably increased. However, by use of a structuraldifference between the porous structure and non-porous structure, asdescribed in this example, it is unnecessary to increase the number oflayers more than the above background art although depending on itsapplication.

Example 2

Hereinafter, Example 2 of the present invention will be described withreference to FIGS. 2A to 2G.

This example relates to a monolithic multilayered film structureincluding silicon and porous SiO₂ (one-dimensional photonic crystal),and a manufacturing method therefor.

A method of forming the multilayered film structure according to thisexample is explained with reference to FIGS. 2A to 2G. In a first step(step (1)), as shown in FIG. 2A, a porous silicon layer 202 is formed onthe surface of a Si substrate 201 (single-crystal substrate obtainedthrough crystal growth by a Czochralski method etc.) through anodizationas shown in FIG. 2B (step (2)).

In a subsequent step, i.e., step (3) of FIG. 2C, heating is conductedwhile supplying H₂O or O₂ and other such gases (i.e. thermal oxidation),thereby turning the porous silicon layer into a porous SiO₂ layer 203.

For the porous silicon of the present invention with a pore size of 10nm or smaller, the thermal oxidation proceeds in an inner portion of theporous silicon faster than the surface thereof. The thermal oxidation isstopped with the thin silicon layer being left on the surface. Hence, astarting point of the epitaxial growth of silicon in the next step canbe left. It is not always necessary to completely oxidize the innerportion of the porous silicon layer depending on circumstances. This isbecause the porous structure of the present invention is amicrostructure that does not optically scatter light, so the innerportion of SiO₂ and silicon only influences optical characteristics onaverage. Thus, intended reduction in refractive index throughoxidization of silicon into SiO₂, i.e. increase in refractive indexdifference with the epitaxial growth silicon layer can be realizedenough even if the porous structure is not completely turned into SiO₂with the silicon being partly left.

In a subsequent step, i.e. step (4) of FIG. 2D, SiO₂ of the surface ofthe porous SiO₂ layer formed in step (3) is etched off with an HFsolution etc. As a result, as mentioned above, the silicon layer thatremained without being thermally oxidized is exposed on the surface,after which epitaxial growth of silicon is allowed to proceed using thesilicon surface of the surface as a starting point to form an epitaxialgrowth silicon layer 204. The epitaxial growth silicon layer 204 has athickness about twice larger than that of the porous layer but, asdescribed later, is precisely controlled depending on the thicknessratio (duty) between the silicon layer and the porous silicon layer inthe multilayered film.

In a subsequent step, i.e. step (5) of FIG. 2E, the surface of theepitaxial growth silicon layer 204 formed in step (4) is turned intoporous silicon (205) through the anodization. Note that the anodizationis allowed to proceed up to a given thickness from the surface; inshort, part of the epitaxial growth silicon layer remains. Thethicknesses of the epitaxial growth silicon layer and porous layer aredesigned according to a target optical element, and a light wavelengthor light incident angle distribution, and may be changed for each layer.Here, the design is basically made in consideration of the refractiveindex of the silicon layer and the refractive index according to theporosity of the porous silicon layer. Assuming that a refractive indexof silicon is 3.5, an effective refractive index of porous silicon is2.5, and a light wavelength adopted is 1.5 μm, a typical layer thicknessis such that an optical path length in each layer, i.e. a length derivedby multiplying the refractive index by the layer thickness equals ¼ oflight wavelength, leading to the condition that the silicon layerthickness is 0.1 μm and the porous silicon layer thickness is 0.15 μm.

In a subsequent step, step (6) of FIG. 2F, as in step (3), the epitaxialgrowth silicon layer 205 made porous is turned into a porous SiO₂ layer206 through thermal oxidation.

As a subsequent step, the processes of steps (4) to (6) are furtherrepeatedly carried out on the structure completed up to step (6) in thisway, so that one epitaxial growth silicon layer and one porous siliconlayer are added in one cycle. As a result, a multilayered film structure207 including the porous SiO₂ layer and the epitaxial growth siliconlayer is obtained (FIG. 2G).

The multilayered film structure 207 is made up of a multilayered filmincluding silicon and porous SiO₂ and has such an advantage that adifference in refractive index between layers can be further increasedwithout increasing the porosity as compared with a conventionalmultilayered film where a porosity of porous silicon is modulated.Hence, upon manufacturing a high-reflectance mirror or Fabry-Perotresonator using such a multilayered film, it is possible to reduce thenumber of layers and realize reduction in size of an element and in costfor a high-cost laminated structure obtained through epitaxial growth.

Example 3

Hereinafter, Example 3 of the present invention will be described withreference to FIGS. 3A to 3D.

This example relates to a monolithic three-dimensional structure(three-dimensional photonic crystal) where a multilayered film structureincluding silicon and porous silicon or porous SiO₂ has a pattern in anin-plane direction, and a manufacturing method therefor.

This example uses, as a starting point (starting structure), themultilayered film structure (denoted by ‘105’ in FIG. 1E) manufacturedby using the method according to the present invention, for example, themethod of Example 1 above. As schematically shown in a sectional viewand top view of FIGS. 3A and 3B, respectively, a photonic crystalpattern such as a triangular lattice pattern is formed throughphotolithography and ICP dry etching in a direction vertical to the filmsurface of the multilayered film structure so as to pass through plurallayers of the multilayered film (step (1)).

For patterning for forming a through-hole pattern, various methods suchas electron beam (EB) lithography, near-field photolithography, X-raylithography, and ion beam lithography can be used as appropriate asidefrom the photolithography. It is needless to say that an optimum etchingmethod, e.g. ECR or other such methods, can be used for etching asidefrom ICP etching, according to various conditions.

Through this step (1), an in-plane pattern is added to theone-dimensional periodic structure, offering a three-dimensionalphotonic crystal. In short, the periodic structure is formed in thedirection vertical to the surface by the utilization of the refractiveindex difference between the epitaxial growth silicon layer and theporous silicon layer, while the periodic structure is formed in thein-plane direction by the utilization of the refractive index differencebetween the air in the through-hole and the silicon layer or the poroussilicon layer.

In order to increase the refractive index difference in thethree-dimensional periodic structure, the following step (2) shown inFIGS. 3C and 3D can be additionally performed.

In step (2), thermal oxidation is promoted under heating while a gas asa source of oxygen atoms, such as O₂ or H₂O, is caused to flow throughthe through-hole formed in step (1). The thermal oxidation becomes aso-called accelerated oxidization due to an effect of a porous structurein the porous silicon layer, and allows oxidization at a rate about 100times higher than that of the epitaxial growth silicon layer. Hence, theentire porous layer can be oxidized even in such a short period as tooxidize just the surface of the epitaxial growth silicon layer. Thus,the three-dimensional structure of this example includes the epitaxialgrowth silicon and the porous SiO₂ in the thickness direction, making itpossible to increase a refractive index difference in the thicknessdirection. This enables reduction in the number of layers necessary forattaining the same effect, and reduction in size of the element and costfor the high-cost laminated structure obtained through epitaxial growth.

Example 4

Hereinafter, Example 4 of the present invention will be described withreference to FIGS. 4A to 4E.

This example relates to a monolithic three-dimensional structure(three-dimensional photonic crystal) where a multilayered film structureincluding silicon and porous silicon or porous SiO₂ has in-planepatterns different from one layer to another layer, and a manufacturingmethod therefor.

In FIG. 4A, first, a silicon substrate 401 is prepared, and a poroussilicon portion 402 is formed with a given in-plane pattern throughanodization in step (2) (FIG. 4B).

FIGS. 5A to 5G are referenced to describe an example of a method ofpatterning the porous silicon layer through anodization.

First of all, as an example A, as shown in FIG. 5A, a silicon substrateis prepared in step (A1). Then, as shown in FIG. 5B, a resist is appliedonto the silicon surface in step (A2), followed by patterning of theresist through a photolithographic technique. The pattern is formed soas to outline a portion to be anodized. In step (A3), an electric fieldis applied in an HF solution with the resist pattern being left thereonfor anodization, with the result that the silicon surface covered withthe resist is insulated and protected against the injection of carriersand intrusion of the HF solution, and hence is not made porous throughanodization (FIG. 5C). Accordingly, only portions not covered with theresist are made porous. After the resist is removed in step (A4), thesilicon substrate is obtained where the porous portions are patterned(FIG. 5D).

Next, as an example B, in step (B2) of FIG. 5F where anodization isperformed in the HF solution on the silicon substrate prepared in stepB1 as shown in FIG. 5E, unlike the general method of arranging a meshelectrode apart from the substrate and uniformly applying an electricfield in an in-plane direction of the substrate as shown in FIG. 13,such a structure is used that a microelectrode is provided in thevicinity of a pole of the substrate and an electric field intensityvaries in an in-plane direction. In other words, anodization proceeds inonly portions applied with a high-intensity electric field to make theportions porous. In addition to patterning through local application ofthe electric field, the light may be locally applied to generatephotocarriers, and the photocarriers may serve to locally promote theanodization.

For resist patterning for anodization, various methods such as electronbeam (EB) lithography, near-field photolithography, X-ray lithography,and ion beam lithography can be used as appropriate aside from thephotolithography.

The pattern of the porous silicon in this example may be set to apattern A shown in FIG. 6A. For example, in FIG. 6A, the porous portionis denoted by ‘601’. Regarding the size of pattern A, the lightwavelength adopted is set to 1.5 μm, the layer thickness is set to about0.25 μm, and the pattern cycle is set to about 0.7 μm. Note that thesize corresponds to about ½ to ¼ of the above respective values in thecase of using a visible light, for example, but the same structure canbe used without any change.

Next, in FIG. 4C, the epitaxial growth is allowed to proceed startingfrom the surface made of the porous silicon and silicon to form asilicon layer 403 in step (3). The thickness of the silicon layer is setequal to the thickness of the porous silicon layer formed in step (2)(about 0.25 μm), but may be set to an optimum thickness according tovarious conditions such as its applications, porosities, and in-planepatterns.

Next, in step (4) of FIG. 4D, the porous silicon layer is formed bysubjecting to the in-plane patterning the epitaxial growth silicon layerformed in step (3) through the anodization. Note that as for patterningin this step (4), a pattern B shown in FIG. 6B is formed with adifferent shape from the pattern in step (2). The porous portion isdenoted by ‘603’ as in the pattern A.

Next, in step (5) shown in FIG. 4E, steps (2) to (4) are furtherrepeatedly performed to afford a laminated structure including multiplelayers with an in-plane pattern of the porous silicon portion. Forexample, the layers are laminated to a given number of layers necessaryin terms of optical performance for the photonic crystal, offering athree-dimensional photonic crystal structure. Here, with regard to thepatterns of the respective epitaxial growth silicon layers repeatedlyformed, patterns of FIGS. 6A to 6D are laminated in the order of, forexample, A, B, C, D, A, B, . . . with reference to FIG. 7.

In general, the number of layers necessary for light-confinement controlutilizing the photonic band gap is, for example, about eight. Forexample, in the case of forming a defect at the center forlight-confinement control or the like, a laminated structure of about 16layers is necessary, in which 8 layers are provided on each side of thedetect.

Further, the patterns in those layers can be designated independently ofeach other. It is possible to easily realize the laminated structurehigh in degree of freedom; for example, two types of photonic crystalsdifferent in refractive index period can be made to coexist. Thus, ahigh-performance optical element and system can be attainedthree-dimensionally.

Note that pattern groups A to D of the respective layers shown in FIGS.6A to 6D in this example can be changed in various forms inconsideration of their purposes, materials, patterning devices, etc. asappropriate. Thus, various laminated structures can be formed; forexample, the respective layers with patterns A to D as shown in FIGS. 8Ato 8D are laminated into a laminated structure of FIG. 9, which is adiamond-like structure with a rectangular lattice form, and patterns Ato C of FIGS. 10A to 10C are laminated into a periodic structure with acylindrical triangular lattice form as shown in FIG. 11. In addition,the hole of the pattern is deviated from the periodic position, the holesize is changed, or the hole is locally eliminated, so positions of thedefective structures may be designated three-dimensionally in theperiodic refractive index profile pattern to form the laminatedstructure with ease.

Also, in the above description, the adjacent layers in the laminatedstructure may have different patterns. In practice, the respectivelayers may have patterns different only in translational deviation inmany cases. Also, layers having no in-plane patterns may be laminatedaccording to its application and design.

Further, with the method of the present invention, it is possible toreduce the thickness of each layer to 1 nm to 10 nm. By making use ofits advantage, one unit of the refractive index profile of the photoniccrystal in the lamination direction is formed with plural epitaxialgrowth silicon layers, so the structure in terms of the layer thicknessdirection can be changed within the one unit of the refractive indexprofile. FIGS. 12A and 12B show an example thereof. Here, the patternstructure in one layer shown in FIG. 12A is obtained by laminatingplural layers with the patterns being changed as shown in FIG. 12B. Morespecifically, in the case of forming the spherical or cylindricalstructure, as shown in FIG. 12B, the structure can be replaced by theapproximate multilayered film structure. Thus, given patterns are formedin the respective thin-film layers to be laminated, thereby producingthe approximate form. The thickness of the thin film can be set to 1 nmto 10 nm as mentioned above, so that in the case of forming anapproximate structure of the spherical structure having a diameter of200 nm with the laminated structure, twenty layers of 10 nm-thick thinfilms are laminated to obtain the approximate structure with accuracy.

Also, as for the three-dimensional photonic crystal produced by usingthe porous silicon or porous SiO₂ and epitaxial growth silicon of thisexample shown in FIG. 19A, the accelerated oxidation described in theabove example may be used to make the porous silicon into porous SiO₂.Thus, the three-dimensional photonic crystal including the porous SiO₂and the epitaxial growth silicon can be manufactured (FIG. 19B).

In addition, as for the three-dimensional photonic crystal produced byusing the porous silicon or porous SiO₂, and the epitaxial growthsilicon of this example, the porous silicon or porous SiO₂ isselectively etched relative to the epitaxial growth silicon, making itpossible to manufacture the three-dimensional photonic crystal includingthe air and the epitaxial growth silicon (FIG. 19C).

The selective etching conditions for the porous layer are listed belowby way of example:

Etching Condition:

-   Solution: HF/H₂O₂-   Etching selection ratio:crystal layer:porous layer=1:100,000

In this case, as shown in FIG. 16, hydrogen annealing may beadditionally carried out after etching. Thus, the surface of theepitaxial growth silicon after etching can be made smooth up to anatomic level, improving a performance of the crystal as an opticalelement. Annealing conditions are listed below by way of example.

Annealing Condition:

-   Gas: 100% H₂-   Temperature: 1,050° C.

Example 5

Hereinafter, Example 5 (PC laser) of the present invention will bedescribed with reference to the accompanying drawings. This examplerelates to an example where a laser device is composed by using the one-or three-dimensional photonic crystal manufactured by the method of thepresent invention.

FIG. 15A is a schematic diagram showing a photonic crystal laser where alaser medium layer 1503 is formed in a planer form. In FIG. 15A, thelaser medium layer 1503 is interposed between a lower three-dimensionalphotonic crystal 1501 and an upper three-dimensional photonic crystal1502.

There are two processes adaptable to such arrangement, that is, (1) aprocess for laminating the layers in order from the lower photoniccrystal using the method described in Example 4 above in which the lasermedium layer 1503 is also regularly laminated through epitaxial growthetc., and (2) a process for manufacturing the upper and lower photoniccrystals in advance by the method of Example 4 and then combining thephotonic crystals with the laser medium layer 1503. At this time, theperiodic structure, or the periodic structure and defective structurecan be formed by patterning the laser medium layer 1503 itself.

In the laser medium layer 1503, a current injection wiring (not shown)or photoexcitation optical system is arranged. Based on the principle oflaser, laser oscillation is generated with the upper photonic crystal1501 and the lower photonic crystal 1502 used as a resonator, i.e.band-narrowing elements. The laser oscillation mode is of an oscillationmode of the DFB type, i.e. an oscillation mode of the so-called photonicband-edge type, in the case of a periodic structure where the upperphotonic crystal 1501 and the lower photonic crystal 1502 have nodefects introduced therein. As a result, the mode involves relativelywide spatial divergence, in short, divergence over plural periods of thephotonic crystal. Also, in adjacent layers next to the laser mediumlayer 1503 of the upper photonic crystal 1501 and the lower photoniccrystal 1502, as shown in FIG. 15B, a defect 1504 may be introduced. Inthis case, as a whole, the laser resonator emits light with a localmode, in short, an oscillation mode with a divergence over about onecycle of the photonic crystal.

The laser medium layer 1503 may be formed of a host material includingan organic pigment such as coumarin, rhodamine, DCM, or Alq₃, or otherpigments. A compound semiconductor containing a ternary or quaternarymixed crystal such as GaAs, InP, InGaN, InGaAs, or InGaAlP can be usedaccording to its application. The laser medium layer may have aninternal structure selected from various structures such as multiplequantum well structure and quantum dot structure as appropriate.

Further, an Er ion can be used as a laser medium as well. In this case,it is possible to locally change a portion to be implanted with Er ionsinto porous silicon in advance or into amorphous silicon through laserirradiation in order not to easily deactivate the Er ions even at normaltemperature.

Meanwhile, as an excitation source, the following ones may be used asappropriate. That is, an electron or hole transporting material such asAlq₃ or TPD widely used in a so-called organic EL element may be used.Alternatively, current injection may be performed through an electrodeformed of ITO, MgAg, or the like. Besides, light excitation may beinduced using an N₂ gas laser, and Nd:YAG harmonic and blue/UVsemiconductor laser. In this example, aside from the above, the lasermedium 1602 is arranged in a dot shape in the three-dimensional photoniccrystal 1601 to compose a dot-defect resonator as shown in FIG. 16. Notethat FIG. 16 is a schematic diagram showing the section taken in thedirection along the paper surface at the position at which the lasermedium 1602 is arranged.

As mentioned above, the three-dimensional photonic crystal laser isrealized using a technique of forming a periodic structure by repeatingthe anodization and epitaxial growth according to the present invention.With the three-dimensional photonic crystal resonator having a highperformance and little loss, a light-emitting element can be produced,which enables oscillation with a wavelength (e.g. oscillation with agreen light wavelength hardly realized with the semiconductor laser)which has been difficult so far, or oscillation with a micro mode.

In the above example, the three-dimensional photonic crystal is used.However, the present invention is not limited thereto but needless tosay, allows various forms, for example, a surface light emitting laseras shown in FIG. 17, which is composed by using multilayered films (1701and 1702) made of the one-dimensional photonic crystal of Example 1 ofthe present invention and bonding them so as to sandwich an active layer1703.

Further, the laser structure according to this example is, of course,used as not only a laser element but also as an optical switch with highefficiency due to a reinforcing effect resulting from the interactionbetween light and non-linear optical material in the photonic crystalstructure of the present invention, as readily understood from the factthat the laser medium itself is a non-linear optical material.

The present invention is not limited to the examples described above,but allows various changes in sequence flows etc. without departing fromthe gist of the present invention.

The present invention is not particularly limited to the aforementionedsilicon material but can be similarly implemented by using a III–Vcompound semiconductors such as GaAs, Ge, GaP, AlGaAs, InGaAs, InAs,GaInNAs, InGaP, or InP, or II–VI compound semiconductors such as ZnSe,ZnS, CdSe, or CdS, or a combination of an epitaxial growth..material anda substrate material having similar lattice constants and/or linearexpansion coefficients, for example, GaAs and Ge.

Also, the principle of the present invention can be employed for a widerapplication range, and the present invention is not limited tosubstantially the periodic refractive index profile pattern but isapplicable to manufacture of an optical element of a three-dimensionalstructure having non-periodic or random refractive index profilepattern.

As described in Examples 1 to 5 above, the following effects can beexpected based on those examples.

First, with the method for manufacturing a monolithic optical whichrepeats anodization for forming porous structure and epitaxial growth,it is possible to offer a nanophotonic element such as a photoniccrystal with a higher quality than conventional ones, in the form oflaminated plural layers with a large area. Also, the entire laminatedstructure or part of the layers is subjected to patterning in anin-plane direction, whereby defects can be freely arranged relative tothe three-dimensional optical periodic structure or three-dimensionalperiodic structure.

In addition, the optical element obtained with the above method is madeup of a laminate of a porous layer having a relatively low refractiveindex depending on the porosity or a layer formed by oxidizing suchporous layer to have a lower refractive index, and a crystal layerhaving as large a refractive index as possible. Thus, there is a largedifference in refractive index between the layers. As a result,predetermined characteristics can be attained even by a relatively smallnumber of layers. In addition, as an optical element where an opticalwaveguide is formed in the layer as well, the light can be readilyconfined within the optical waveguide. Further, the pores of the porouslayer are not coupled to each other between the layers, whereby anelement can be obtained with a relatively uniform crystal layer in termsof optical characteristics and little optical loss.

Example 6

Hereinafter, an optical element of this example will be described withreference to FIGS. 21A to 21C.

In FIGS. 21A to 21C, laminated on a silicon substrate 21101 is a cladlayer 21102 of porous silicon maintaining its crystallinity (crystalorientation) at the boundary. A porous silicon waveguide layer 21103which includes therein epitaxial growth silicon waveguides 21104 and21105 intercrossing each other is laminated on the clad layer 21102.

Next, a porous silicon spacer layer 21106 which adjusts the couplingstrength between the waveguide and resonator is laminated thereon. Aporous silicon resonator layer 21107 which includes therein a microringresonator 21108 of epitaxial growth silicon is further laminatedthereon. In this way, a three-dimensional element structure is formed.

Next, functions of this optical element are described. This opticalelement is such that a wavelength filter is made up of a waveguidecircuit. In other words, it is of such an element that selective opticalcoupling in the two waveguides 21104 and 21105 is carried out. In thiscase, meant by the “selective coupling” is, so to say a selectivitydepending on the wavelength. For example, when plural lights havingdifferent wavelengths are guided into the waveguide 21104, a lighthaving a given wavelength is only allowed to couple to the waveguide21105, and light energy is transferred from the waveguide 21104 to thewaveguide 21105. As a result, the light having the given wavelength isemitted from the waveguide 21105, while the other lights having otherwavelengths are not coupled to the waveguide 21105 and thus travelsalong the waveguide 21104 to be emitted.

The microring resonator is responsible for the wavelength selectivity.The microring resonator has such a property that light is guided tocause revolution, and the resonator is firmly coupled to the lighthaving such a wavelength as to meet conditions that light having made Nrevolutions and light having made (N+1) revolutions (where N is aninteger) have phases matched with each other to mutually enhance lightintensity.

Actual conditions for coupling the waveguides 21104 and 21105 and theresonator 21108, and wavelength selection conditions are determinedaccording to various parameters such as the configuration (distance)between the resonator and waveguides inclusive of the thickness of thespacer layer, the refractive index of the spacer layer, and the spatialconfinement mode inclusive of the amplitude profile of a lightpenetrating to a clad out of the lights propagating in the waveguide orresonator.

Also, for such element where the waveguide is coupled through theresonator, the smaller the loss of the resonator, the higher thewavelength selectivity of the resonator. Accordingly, as one featurethereof, the narrow-band wavelength selection filter can be realized. Asanother feature thereof, the transmission band interval is widened byreducing the diameter of the resonator, enhancing the wavelengthselectivity in a broad sense.

Next, the respective layers are described regarding their structureparameters and physical properties.

First, the silicon substrate 21101 is composed of a single-crystalsilicon formed through crystal growth by the Czochralski method etc.,which, for example, has a refractive index of about 3.5 with respect toa light having a wavelength of 1.5 μm.

Next, the clad layer is formed with a thickness of about 1 μm. The cladlayer is formed of porous silicon with its pore size far smaller thanthe light wavelength adopted. For example, the clad layer is formed soas to have a pore size equivalent to about 1/100 of the light wavelengthadopted. This structure prevents anisotropic scattering or diffractionof light but influences only average optical properties. In thisexample, the clad layer is formed for the light wavelength of 1.5 μm.The pore size is set to about 2 nm. On the other hand, the porosity inthis example is about 80%. In other words, it is formed such that thevolume ratio between the silicon and the air is about 2:8.

In this case, the average effective refractive index of the poroussilicon layer is calculated roughly based on the following expression(1) and found to be about 1.5.n _(eff) =n _(air) ×x _(air) +n _(si) ×x _(si)  (1)where n_(eff) represents an effective refractive index, n_(air)represents a refractive index of the air filled in the pore, x_(air)represents a porosity, n_(si) represents a refractive index of silicon,and x_(si) represents a volume ratio of silicon and equals (1−x_(air)).In this example is found,n _(eff)=1.0×0.8+3.5×0.2=1.5

This value is equivalent to that of SiO₂ in a buried-oxide (BOX) layerof a conventionally used SOI wafer. A difference Δn from the refractiveindex of the epitaxial growth silicon in the waveguide layer as a corelayer, i.e. about 3.5, is about 2, enabling strong light-confinement.The fact that the strong light-confinement is enabled means that thelight can be localized into a smaller volume when it is used as anoptical waveguide or resonator, realizing miniaturization and highintegration degree of the optical elements.

Next, the waveguide layer 21103 is formed with a thickness of about 0.2μm, in which the waveguides 21104 and 21105 each have a width of about0.2 μm. Those waveguides are composed of three porous silicon layers of(1) the above clad layer forming a lower layer, (2) a waveguide layerforming the side face, and (3) the following spacer layer forming anupper layer; the light-confinement is thus realized with a refractiveindex difference Δn of about 2, so the waveguides guide light straight,respectively. Due to the large refractive index difference, even a thinwaveguide having a width of 1 μm or smaller can guide light with smallloss.

Next, the spacer layer 21106 is formed with a thickness of about 0.1 μm.The thickness is a parameter for controlling the coupling strengthbetween the upper straight waveguides 21104 and 21105, and the resonator21108 and is determined according to the desired performance of theoptical element. Also, the porosity of the porous silicon is about 80%in this example. The porosity is also a parameter for controlling thecoupling strength between the above waveguides and the resonator via theabove effective refractive index, and thus is determined inconsideration of the layer thickness according to the desiredperformance of the optical element.

Next, the resonator layer 21107 is formed with a thickness of about 0.2μm. The diameter of the microring resonator is set to about 2 μm in thisexample.

A feature of the present invention resides in the realization of such amicro resonator or fine waveguide. Thus, in addition to the advantage interms of the high wavelength selectivity, for example, extremely highdensity integration can be realized in configuring a circuit networkincluding a number of optical elements with this optical element used asa unit element.

Subsequently, the manufacturing method of the present invention, whichrealizes the aforementioned micro optical element, will be describedwith reference to FIGS. 22A to 22H. First, in step (1) in FIG. 22A, asilicon substrate 22101 is prepared. In step (2), a silicon layer 22102having a porous structure is formed on the surface of the siliconsubstrate 22101 through anodization (FIG. 22B). Note that in thefollowing description, the anodization of the present invention ispreferably effected at a low temperature not higher than 0° C., e.g.about −20° C. This aims to obtaining a smooth surface at the boundary ofthe porous silicon formed through the anodization and suppressingoptical loss, i.e. light scattering etc. Also, the term “porous” used inthe present invention means the condition that the pore size is equal toabout 1/100 or smaller of a wavelength of light used; for example, withrespect to a light having a wavelength of 1.5 μm, a so-calledMesoporous-Si or Microporous-Si having a pore size of 10 nm or smalleris formed. The anodization can be carried out with the device structureas shown in FIG. 23, for example.

In FIG. 23, a silicon wafer (substrate) 23301 is held so as to immerseits epitaxial growth Si layer in a HF solution 23302. A lower supportingmember 23305 and a upper supporting member 23306 hold the wafer throughan O-ring 23303 and a Pt surface electrode 23304. The upper supportingmember 23306 is provided with a HF solution tank communicating with theSi wafer 23301, which is filled with the HF solution 23302. A Pt meshelectrode 23308 is placed in the HF solution 23302. The Pt surfaceelectrode 23304 and the Pt mesh electrode 23308 are connected to ananode 23307 and a cathode 23309, respectively. A predetermined electricfield is applied to the Si wafer through the HF solution 23302 on thecathode side and through the Si substrate rear surface on the anode sidefor carrier injection. Note that the structure for anodization is notlimited to that of this example, but generally-employed various methodscan be used as appropriate.

Anodization conditions are listed below.

-   Starting wafer: p+(100)Si 0.01 Ωcm-   Solution: HF, C₂H₅OH, H₂O-   Anodization current: 150 mA/cm³

Also, as for the anodization method, it is possible to collectivelyprocess plural wafers with a device structure as shown in FIGS. 4A to4E. This step can be thus performed at a low cost.

Also, the pore size, density, and thickness of the porous silicon layercan be controlled over a wide range depending on the composition of ananodization solution, anodization current, conductivity type of thesubstrate, and conductivity thereof. Also, platinum having an extremelyhigh resistance to hydrofluoric acid or metal covered with platinum onits surface is used as an electrode. In the case of collectivelyanodizing plural wafers into porous layers, as shown in FIG. 24, theanodization solution itself in contact with both sides of the wafer actsas an electrode. This enables uniform contact and enhancescontrollability of a porous layer to be formed.

As described above, the clad layer made of porous silicon is formedthrough anodization.

Subsequently, in step (3), epitaxial growth is promoted starting fromthe silicon surface forming the clad layer surface to form the siliconlayer 22201 (FIG. 22C). In the epitaxial growth, as well known, theporous layer formed by the above method maintains its crystalorientation of the single crystal substrate, and a uniform, pore-freesingle crystal layer can be formed thereon through epitaxial growth.

Here, a description will be given of conditions for forming an epitaxialgrowth Si layer 22203 through the epitaxial growth on the porous Silayer 22202 by the chemical vapor deposition (CVD) method or the like.First, it is important to promote the epitaxial growth in a hydrogenatmosphere. Sealing of the pores in the surface of the porous layer isaccelerated, and the epitaxial layer with high quality is formed thereon(Yonehara et al., “Appl. Phys. Lett.”, Collective Report, September,2002)

Epitaxial growth conditions are listed below.

-   Vapor deposition Temperature: 1,000° C.-   Gas: SiH₄/H₂-   Pressure: 700 Torr

Subsequently, in step (4), the surface of the epitaxial growth siliconfilm formed in step (3) undergoes anodization to obtain a porous siliconlayer. At this time, patterning is performed for part of in-planeportions, that is, the waveguides 22104 and 22105 are not made porousbut remain the epitaxial growth silicon (FIG. 22D).

FIGS. 25A to 25G show two examples of a method of realizing anodizationpatterning, with which porous structure is formed at a desired portionalone, as described above. Note that in the following examples, ageneral method of patterning plural portions is described.

First, as an example A, as shown in FIG. 25A, a silicon substrate isprepared in step (A1). Next, as step (A2), a resist is applied to thesilicon surface and patterned by the photolithographic technique (FIG.25B). The pattern is formed so as to outline a portion to be anodized.In step (A3), an electric field is applied in an HF solution with theresist pattern being left thereon for anodization, with the result thatthe silicon surface covered with the resist is insulated and protectedagainst injection of carriers and intrusion of the HF solution, andhence is not made porous through anodization (FIG. 25C). Accordingly,only portions not covered with the resist are made porous. After theresist is removed in step (A4), the silicon substrate is obtained wherethe porous portions are patterned (FIG. 25D).

Note that for resist patterning for anodization, various methods such aselectron beam (EB) lithography, near-field photolithography, X-raylithography, and ion beam lithography can be used as appropriate asidefrom the photolithography.

Next, as an example B, in step (B2) (FIG. 25F) of performing anodizationin a HF solution on a silicon substrate prepared in step (B1) of FIG.25E, unlike the general method of arranging a mesh electrode apart froma substrate and uniformly applying an electric field in an in-planedirection of the substrate as shown in FIGS. 5A to 5G, it is used such astructure that an electrode having a size substantially equal to adesired pattern is provided in the vicinity of a pole of the substrateand an electric field intensity varies in an in-plane direction. Inother words, anodization proceeds in only portions close to the pole,which is applied with a high-intensity electric field to make theportions porous. In addition to patterning through local application ofthe electric field, the light may be locally applied to generatephotocarriers, and the photocarriers may be used to accelerate theanodization.

As a subsequent step in the element manufacturing process, in FIG. 22E,epitaxial growth is promoted in silicon in step (5), as in step (3). Ina subsequent step, i.e. step (6), the silicon is made porous throughanodization into the spacer layer 22106 (FIG. 22F).

Next, a silicon layer for forming a resonator is laminated throughepitaxial growth again (FIG. 22G). Then, as described in step (4), thesilicon is made porous through anodization except for a portioncorresponding to a microring resonator (FIG. 22H) at this time.

As described above, through steps (1) to (8) shown in FIGS. 22A to 22H,an optical element as the wavelength selection optical circuit of thisexample is formed as a monolithic silicon structure.

Further, in this example, the porous silicon is thermally oxidized at arate about 100 times higher than the non-porous silicon portion(accelerated oxidation). Based on this, the porous silicon of the cladlayer or of clad portion of the waveguide or resonator is subjected tothermal oxidation to turn it into porous SiO₂, thereby enabling a lowerrefractive index.

Note that a supplemental description of the background art will bebriefly added here.

In recent years, a research about how to utilize SOI wafers to 2D slabtype optical elements is being accelerated. The 2D slab type refers tosuch a type that clad layers having a low refractive index sandwich acore layer having a high refractive index to confine light into the corelayer having a high refractive index and propagate the light therein toachieve light-confinement in a non-periodic direction.

In the case of using SOI wafers, the following functions are imparted tothe SOI structure. First of all, SiO₂ formed on an Si substrate (BOXlayer: buried oxide layer) is used for a clad, and Si formed thereon(SOI layer: silicon on insulator) is used for a core (see, Noutomi“Photonic Crystal Slab using SOI Slab”, Appl. Phys. Lett., vol. 72, No.7. 2003).

In this case, the slab thickness, that is, the thickness of the corelayer, depends on the conditions under which the light can have anelectromagnetic mode in the thickness direction. In particular, whenonly a single mode is allowed, an optical path length derived bymultiplying the slab thickness by the refractive index is equal to about½ of the wavelength. In other words, one round-trip optical path lengthapproximates one wavelength. This corresponds to such a condition thatthe smallest thickness is used to allow light that has made oneround-trip to interfere with light that has made several round trips,thereby strengthening each other. In practice, in consideration of thepenetration of the light to the clad layer, the thickness is calculated(see, Koshiba “Optical Waveguide Analysis”, Asakura Shoten, 1990).

In the case of using such an Si material, there are the followingadvantages. That is, (1) manufacturing techniques for SOI wafers aredeveloped to a practical level and its precision is secured, and (2)sophisticated techniques of the Si process can be used for patterningtechniques for forming a periodic pattern in an SOI layer being a corelayer.

Among 2D slab elements using SOI, the 2D slab photonic crystal is themost famous subject of research (see Sato “Photonic Crystal Techniqueand its Application” p. 229, 2002, CMC Publishing), but there is an Sithin waveguide that is expected to produce the same effect as in thephotonic crystal. In this case as well, research and development areunder way on light-confinement into a thin micro waveguide having athickness of 1 μm or less as in the 2D slab photonic crystal, utilizinga large difference in refractive index between Si and SiO₂, or a devicesuch as a bent waveguide element having a small curvature (see, Kawakamiet al., “Photonic Crystal Technique and its Application”, pp. 252, 257,and 258, 2002, CMC publishing Co., Ltd.).

Also, in recent years, an attempt has been made to produce a microrouting element in which the ring resonator and straight waveguide arecombined, which has been under study so far (see, Kokubun “MicroringResonator Optical Routing Element”, Appl. Phys., vol. 72, No. 11, 2003)by using the above-described waveguide system having a large refractiveindex difference which uses a SOI wafer.

In the case of using the SOI wafer for the 2D slab type photonic crystalor thin waveguide, the BOX layer is required to have a relatively largethickness, for example, desirably 1 μm or larger. This is due to thelight-confinement condition. That is, in the case of light-confinementin the core layer, as mentioned above, the light penetrates to the cladlayer, and if the clad layer is thin, the evanescent mode of penetratedlight is coupled to a radiation mode to the substrate, resulting in theradiation loss in the substrate direction.

The publication of Kawakami et al. describes a calculation exampleregarding the requisite thickness of the BOX layer with the allowableloss set to −40 dB.

In manufacturing SOI having a thick BOX layer having a thickness of 1 μmor larger, it is necessary to employ a so-called bonding typemanufacturing method. As for bonded wafers of this type, there arewafers reported in Cited Document A (Celler and Yasuda “CurrentCondition of MEMS SOI wafer” May 2002, Electron Technology) and CitedDocument B (Iyer and Auberton-Herve “SILICON WAFER BOUNDING TECHNOLOGYfor VLSI and MEMS applications”, EMIS PROCESSING-SERIES 1, ISBN 0 85296039 5, 2002, The Institution of Electrical Engineers) and these havebeen put into practical use.

However, for manufacturing such bonded wafers, a bonding step should beinvolved. Besides, plural starting wafers should pass throughcomplicated steps such as a seed wafer cutting step accompanied with anH⁺ ion implantation step. As a result, as compared with general Siwafers etc., special structure and process are necessary for elements tobe formed therewith. In addition, the wafer itself costs high. Itsapplication has been undesirably limited to a high-value addedsemiconductor logic circuit such as a CPU worth high cost.

Further, in the case of manufacturing a routing element in which a ringresonator and a straight waveguide are combined by using a SOIstructure, as for a circuit configuration composed of athree-dimensional waveguide and resonator having two or more corelayers, which is said to produce a highly effective element, there issuch a disadvantage that the SOI wafer involves the need to bond two SOIwafers after patterning on an intermediate layer (wafer bonding) andrequires a higher cost.

The above problems can be solved by applying Example 6 of the presentinvention.

Example 7

This example relates to a circuit network realized by the presentinvention which is formed by combining plural three-dimensional elementswhere a thin waveguide and a microring resonator are optically coupled,in an in-plane direction. In particular, this example has the featurethat patterning is carried with varying selectivity of the circuit byvarying porosity of the spacer layer which contributes to couplingbetween the optical resonator and waveguide for each element (or eacharea). Hereinafter, referring to FIG. 26, an optical circuit element ofthis example will be described.

FIG. 26 is a top view of the optical circuit element of this example.However, portions assigned with reference numerals 26603 to 26611 areshown as perspective images.

First, in this optical circuit element, nine (3×3) light selectionelements including two waveguides and one microring resonator of Example1 are arranged two-dimensionally. The elements are connected at theboundaries of the waveguides as shown; in the figure in which threewaveguides are arranged lengthwise and crosswise, respectively. As for amanufacturing method therefor, six waveguides in total and ninemicroring resonators are formed collectively in each plane throughpatterning according to the method of Example 1. The microring resonatorof this example is formed with absolutely the same size and similarpositional relationship with the waveguide (distance therefrom).

Next, description is given of a portion of this example making use ofthe present invention, that is, a spacer layer which couples thewaveguide layer and the resonator layer. Regarding the spacer layer,nine regions in the same plane are collectively formed similar to thewaveguide layer and resonator layer. Note that upon the formation, theanodization conditions may be changed for each region so as to obtainrespective predetermined porosities. A method of anodizing the regionsfor the different porosities is similar to the example B of thepatterning method described above with reference to FIGS. 25E to 25G,that is, a method of anodizing the regions using plural electrodes,which is realized by changing an applied electric field for each of theplural electrodes.

Nine light selection elements formed of the spacer layer havingdifferent porosities for the regions have a coupling strength differentfrom the waveguide since, even if the sizes and positions of themicroring resonators are the same, the effective refractive indexes ofthe spacer layers are different. Thus, the nine light selection elements(regions) which form coupling between the waveguides can have differentcharacteristics

An important effect of this example described so far is given below.That is, only one condition needs to be met for a factor such as theposition or size of the microring portion depending on theminiaturization process. If the difference in element characteristics isattained by slightly changing the size or position of the microringresonator, it is necessary to precisely control them. However, in thisexample, a single condition is set therefor, thereby enabling simplerand stable control.

The present invention is not limited to the above-mentioned examples,but allows various changes in sequence flows etc. without departing fromthe gist of the present invention.

For example, in Example 1, the waveguides extending in two differentdirections are connected and arranged in the same layer, but may beformed in different layers. For example, as shown in FIGS. 27A and 27B,one of the waveguides may coexist in the resonator layer. Alternatively,as shown in FIGS. 27C and 27D, the two waveguides and the resonator maybe arranged in different layers and separated through the spacer layersto be optically coupled.

The present invention is not limited to the aforementioned siliconmaterial but can be similarly implemented by using a III–V compoundsemiconductor such as GaAs, Ge, GaP, AlGaAs, InGaAs, InAs, GaInNAs,InGaP, or InP, or II–VI semiconductor such as CdSe or CdS, or acombination of an epitaxial growth material and a substrate materialhaving similar lattice constants and/or linear expansion coefficients,such as GaAs and Ge.

The optical element of the present invention is formed using amonolithic optical element structure where a porous silicon and porousSiO₂ prepared through anodization as well as a silicon layer formedthrough epitaxial growth are laminated after in-plane patterning, andhas a high performance and precision. The optical element can be usedfor optical communication or information processors using light.

INDUSTRIAL APPLICABILITY

The optical element according to the present invention is applicable ina wide range of applications, such as an optical communication includingimage display, image transfer and data communication, an informationprocessor using light, and in addition a sensor and detection system fordetecting various types of information such as image information and bioinformation with high sensitivity.

This application claims priority from Japanese Patent Application No.2003-434555 filed Dec. 26, 2003, and Japanese Patent Application No.2004-033507 filed on Feb. 10, 2004 which are hereby incorporated byreference herein.

1. An optical element, comprising: a first porous region; a secondporous region; and a non-porous region formed between the first porousregion and the second porous region, the non-porous region having arefractive index higher than a refractive index of the first porousregion, wherein a first layer including the first porous region isformed, a second layer including the second porous region is formed, anda third layer including the non-porous region is formed between thefirst layer and the second layer and has in its in-plane direction aregion with a refractive index different from the refractive index ofthe non-porous region, wherein the non-porous region functions as anoptical waveguide, and a spacer layer of a porous structure is formedbetween the second layer and the third layer, wherein the spacer layerincludes in its in-plane direction a plurality of regions different inrefractive index.
 2. The optical element according to claim 1, whereinthe region in the third layer with the refractive index different fromthe refractive index of the non-porous region has a porous structure. 3.A light emitting element, comprising: an optical resonator comprised ofthe optical element according to claim 1; and a light emitting substanceprovided to at least one of an inner portion and vicinity of the opticalresonator.
 4. An information processor, comprising: the optical elementaccording to claim 1; and a light emitting portion.